With regard to a radio transmitting device, the non-linearity of a power amplifier (PA) to amplify radio frequency (RF) signals has played a huge part in distorting RF signals to be transmitted. The distortion of the RF signal generates leakage power leaked to the outside of a desired frequency band used for transmission. The linear modulation has become a mainstream in the recent radio communication standard in order to improve the spectral efficiency. In the case of the linear modulation, a stringent limitation is set on the leakage power out of a desired band which arises due to the distortion of RF signals. Therefore, it is an important technical problem in the radio transmitting device to suppress the distortion of the RF signal to be transmitted.
On the other hand, toward the realization of further high-speed radio communication, the carrier aggregation (CA) technology has recently been used by which a plurality of fragmented frequency bands are gathered and utilized (see Patent Literature 1, for example). The CA technology makes it possible to secure a wide band by bundling a plurality of frequency bands and speed up transmission rates. In addition, it becomes possible by applying the CA technology to perform communication responding to a situation that discrete frequency bands are allocated to a plurality of operators or that a frequency band is shared.
The CA technology is classified according to frequency allocation of each carrier. As one of them, there is an Inter-band Non-contiguous CA mode in which each carrier frequency is allocated so that a difference between respective carrier frequencies Δf may become sufficiently larger than a modulation bandwidth fee of a RF signal used by each carrier. This CA mode technology makes it possible to improve the stability of the communication by performing communication simultaneously using a plurality of carrier frequencies whose propagation characteristics differ from each other.
A radio communications system using the above-mentioned CA technology requires a radio transmitting device and a radio transmitting method by which RF signals in a plurality of bands are transmitted. The above-mentioned radio transmitting device also requires suppressing the distortion of the RF signal. It is desirable in terms of making a device smaller and at lower cost that the radio transmitting device compatible with the CA technology should be able to amplify and transmit RF signals in a plurality of bands by means of a single power amplifier.
Patent Literature 2 describes an example of the radio transmitting device which transmits RF signals in a plurality of bands. FIG. 16 illustrates a configuration of a related radio transmitting device described in Patent Literature 2. The related radio transmitting device 100 is composed of two blocks of a dual-band transmitter 130 and a pre-distortion unit 120.
The dual-band transmitter 130 has the function of simultaneously transmitting RF signals with the carrier frequency fc1 (band 1) and RF signals with the carrier frequency fc2 (band 2). A baseband signal 115A corresponding to the band 1 is routed through a low pass filter 135A and converted into a RF signal with the carrier frequency fc1 in a frequency converter which is composed of a local oscillation (LO) signal generator 140A and a mixer 145A. A baseband signal 115B corresponding to the band 2 is routed through a low pass filter 135B and converted into a RF signal with the carrier frequency fc2 in a the frequency converter which is composed of a local oscillation (LO) signal generator 140B and a mixer 145B. The RF signal with the carrier frequency fc1 and that with the fc2 are synthesized in a power combiner 150 and then inputted into a power amplifier 160. The power amplifier 160 simultaneously amplifies the RF signal with the carrier frequency fc1 and that with the fc2, and outputs them as a RF signal 170.
The pre-distortion unit 120 has the function of compensating the distortion of the RF output signal 170 which is composed of the band 1 and the band 2 and is generated in the dual-band transmitter 130. That is to say, the pre-distortion unit 120 has non-linear input-output characteristics opposite to the input-output characteristics of the dual-band transmitter 130, by which it cancels the non-linearity of the dual-band transmitter 130.
In the case of transmitting the dual-band signals simultaneously, a frequency mixing effect arises due to the non-linearity of the dual-band transmitter 130. This makes a component in the band 1 of the RF output signal 170 depend on both of the baseband signal 115A in the band 1 and the baseband signal 115B in the band 2 which are inputted into the dual-band transmitter 130. For the same reason, a component in the band 2 of the RF output signal 170 depends on both of the baseband signal 115A in the band 1 and the baseband signal 115B in the band 2 which are inputted into the dual-band transmitter 130. In order to correct for the frequency mixing effect, a predistorter 125A generates the baseband signal 115A in the band 1 from both of an input baseband signal 110A in the band 1 and an input baseband signal 110B in the band 2, and outputs the baseband signal 115A. Similarly, a predistorter 125B generates the baseband signal 115B in the band 2 from both of the input baseband signal 110A in the band 1 and the input baseband signal 110B in the band 2, and outputs the baseband signal 115B.
[Citation List]
[Patent Literature]
[PTL 1]
Japanese Patent Application Laid-Open Publication No. 2012-216969 (paragraph [0005])
[PTL 2]
United States Patent Application Publication No. 2010/0316157 Description (paragraph [0042] and FIG. 10)